Patent document No. 1 discloses a submersible power generator comprising relatively rotating inner and outer rotational armatures, propellers for driving the inner and outer rotational armatures to rotate relatively to each other, and a casing for accommodating the inner and outer rotational armatures, which submersible power generator comprises a power generation mechanism provided with a pair of front and rear outer rotational armatures and a pair of front and rear inner fixed armatures, a pair of front and rear propellers rotating in directions opposite to each other to rotate the pair of front and rear outer rotational armatures in directions opposite to each other, and a casing for accommodating the pair of front and rear inner fixed armatures and shielding them from an external environment, wherein the submersible power generator is installed under water to operate in a water current, and buoyancy acting on the submersible power generator is larger than gravity acting on the submersible power generator.